There are the cases where it is desirable to switch between unidirectionality and non-directionality depending on sound pickup situation. To be more specific, the unidirectionality is desirable in the case where there is a sound other than the sound to be picked up or in the case of amplification use. As opposed to this, the non-directionality is desirable in the case where there is no sound other than the sound to be picked up in a studio or in the case where there is no need for amplification for instance.
For that purpose, two microphones of unidirectionality and non-directionality should be prepared to use one of them according to the sound pickup situation. However, it is not desirable to do so in terms of cost and carrying. There are also the microphones of which unit portions are replaceable. However, replacement thereof takes time and besides, there are the cases where a contact and the like become damaged while repeating the replacement.
On the other hand, it is possible to switch the unidirectionality to the non-directionality by blocking a rear acoustic terminal of a unidirectional condenser microphone unit. A conventional example thereof will be described by using FIG. 4. FIG. 4A is a front view of the unidirectional condenser microphone unit, FIG. 4B is a sectional view thereof, and FIG. 4C is a sectional view in the case of the non-directionality.
With reference to FIGS. 4A and 4B, the unidirectional condenser microphone unit includes a cylindrical housing 10 having a front acoustic terminal 11 on one end face side of a side directed to a sound source. The housing 10 has a converter 20 for converting a sound wave to an electrical signal according to change in capacitance housed therein.
The converter 20 is configured by oppositely combining a diaphragm 21 set up on a supporter ring 22 with a fixed pole 23 supported by an electrical insulating seat 24 via a spacer not shown. The converter 20 is fixed in the housing 10 by a lock ring 26 for instance.
The seat 24 is provided with an electrode drawing rod 25 connected to the fixed pole 23 via an electric conductor not shown. When this unit is mounted on a microphone body not shown, the electrode drawing rod 25 is connected to a gate of an FET which is an impedance converter provided to the microphone body.
To let the sound wave coming around a back of the housing 10 act on a back face of the diaphragm 21, the fixed pole 23 has a large number of sound passage holes 23a provided thereon, and the seat 24 has a rear acoustic terminal 24a provided thereon.
Thus, the sound wave from the front acoustic terminal 11 acts on the front face of the diaphragm 21 while the sound wave from the rear acoustic terminal 24a acts on the back face of the diaphragm 21. Therefore, the condenser microphone unit operates as unidirectional. As shown in FIG. 4C, however, it is rendered non-directional by blocking the rear acoustic terminal 24a with a seal plate 27 for instance.
FIG. 5 shows a frequency response characteristic in the case of unidirectionality, and FIG. 6 shows the frequency response characteristic in the case of non-directionality. As for the unidirectional condenser microphone unit, capacity of its internal air chamber is in proportion to sensitivity of non-directional components. For that reason, if the rear acoustic terminal 24a is merely blocked as shown in FIG. 4C, the sensitivity is lowered and a frequency response deteriorates as a bidirectional component is no longer taken in. If FIGS. 5 and 6 are compared, the sensitivity is lower by approximately 6 dB and the frequency response is inferior in the case of the non-directionality in comparison with the unidirectionality.